1. Field of the Invention
This invention relates generally to the prevention of particulate contamination and static-discharge in cleanrooms. More specifically, this invention relates to anti-static products used in "cleanrooms", semiconductor fabrication plants, pharmaceutical manufacturing facilities, and other applications and environments where extreme cleanliness must be maintained, and to methods for making such products.
2. Description of the Related Art
Cleanrooms are being used more often in a greater variety of areas. The requirements for maintaining cleanliness in semiconductor fabrication cleanrooms, pharmaceutical manufacturing facilities and similar facilities, for example, are stringent. Products brought into and used in cleanroom environments must be carefully designed and manufactured to avoid the risk of contamination. In semiconductor fabrication cleanrooms, for example, surfaces frequently must be wiped with exceptionally clean wipers and cleaning solution in order to prevent contamination. Other examples of cleanroom products include clothing, gloves and stationery products (i.e., notebooks and writing instruments).
It is well known that particulates can be brought into the cleanroom environment by workers themselves and by the materials which they use. In this regard, items which are subject to abrasion or wear are a cause of special concern since such abrasion and wear can result in particle formation. Cleanrooms are characterized by a special emphasis on the prevention of particulate generation and the removal thereof prior to deposition on cleanroom surfaces and products.
The term "applicator" or "wiper", as used in this specification, is intended to mean a cleaning fabric suitable for use in cleaning surfaces in cleanrooms and the like. Such applicators or wipers are distinguished from tissues and similar materials in that they are extremely clean and have a relatively high degree of wet strength and structural integrity. Accordingly, these products do not disintegrate when used to wipe surfaces, even when dampened or saturated with cleaning liquids.
Cleanroom products used in sensitive areas, such as semiconductor fabrication cleanrooms and pharmaceutical manufacturing facilities, are carefully selected for characteristics such as particle emission levels, levels of ionic contaminants, adsorptiveness, resistance to attack or degradation by wear or exposure to cleaning materials, and lack of attack by or degradation by biocides.
The contamination which is to be controlled is often called "microcontamination" because it consists of small physical contaminants, such as particulate matter of a size between that of bacteria and viruses, and chemical contaminants in very low concentrations, typically measured in parts per million or parts per billion.
The contaminants usually are of three types including (1) particles, (2) ions and (3) "extractables", which are impurities leached from the fibers of the wiper, for example.
Loose particles 100 micrometers and smaller in size are an anathema to obtaining high production yields and reliable semiconductor devices. Therefore, wipers, cleaning materials and other products used in cleanrooms should emit as small a number of loose particles as possible. Similarly, ions and "extractables" are to be minimized since each interferes with the exacting process of semiconductor manufacturing.
Such requirements have been met by the provision of specially fabricated products designed to emit very few loose particles or ions, while maintaining structural integrity when used. For example, cleanroom wipers that are wetted with cleaning solution and used to wipe the surfaces to be cleaned.
Various cleanroom products have been developed for use in cleanroom environments. See, for example, U.S. Pat. No. 4,888,229 to Paley et al.; U.S. Pat. No. 5,229,181 to Daiber et al.; and U.S. Pat. No. 5,271,995 to Paley. The disclosures of those patents hereby are incorporated herein by reference.
However, in some cleanroom environments, not only is it necessary to maintain a "clean" environment, it is often also necessary to prevent static discharge. The problem of static electricity has become an ever increasing problem where, for example, sensitive electronic equipment is being manufactured. Basically, static is created when two similar materials are rubbed together and then separated. One object tends to give up electrons whereas the other tends to accumulate them, thereby leaving the former with a positive static charge and the latter with a negative charge. When oppositely charged objects contact each other, a static shock is created which corrects the imbalance.
Control of static electricity can be critical in many industrial or commercial settings where an undesired electrostatic discharge (ESD) or spark can result in serious damage. For example, in explosive environments such as in grain elevators or in flammable environments such as an oil drilling rig or refinery, a spark can be extremely dangerous. In addition, static discharge can damage sensitive integrated circuits. Therefore, such circuits must be safeguarded during their manufacture.
One particularly significant problem is the fact that the average person will not sense a discharge of less than 3500 volts. Since many electronic devices can be damaged by potentials far lower than 3500 volts, such devices can be damaged unknowingly and incorporated into the final product. As a result, the final product will be defective, without anyone knowing it.
Insulating materials can often be a source of static discharge, particularly those having a relatively high value of surface resistance on the order of 10.sup.16 .OMEGA.(Ohms). Such insulated materials should be modified to reduce the risk of static discharge. In the past, this has been accomplished by increasing the electrical conductivity of the products. Such an increase in conductivity allows the product to dissipate the static electrical buildup.
Some prior anti-static cleanroom wipers have been made by incorporating fibers having a higher degree of electrical conductivity. Certain types of carbon and metal fibers are inherently conductive, and therefore the incorporation of such fibers increases the conductivity of the textile material so that the previously static materials can be rendered static dissipative. U.S. Pat. No. 5,324,579 to Sassa et al. discloses a non-woven textile material made useful for dissipating static electric charges by incorporating an electrically conductive fiber.
However, the use of such fibers has many disadvantages. For example, the conductive fibers only allow for increasing conductivity in the direction of the fiber. Accordingly, the resultant product would have different conductivities in different areas and in different directions. Furthermore, conductive fibers may be relatively brittle when compared to the majority of fibers used in a wiper product and therefore have a tendency to break when flexed. Such breakage not only results in a reduction in the static dissipative capability of the material, but also provides a source of contamination to the cleanroom environment.